In various applications, such as refuse compactors, baling equipment, and the like, fluid actuated linear rams are used to compress a product for transportation, storage, or further processing. Oftentimes it is necessary for the ram to travel a considerable distance in a precompression or non-compression mode, wherein the ram is being extended to close the gap between its retracted position and the product to be compressed, or the ram is being retracted to allow introduction of additional product. In a conventional application by way of example, an 8" diameter ram may be moved over a 150 inch stroke with only the last 15 inches in the compression region. In a typical single acting cylinder the fluid pump would deliver pressurized fluid to the ram over the entire stroke, yet compression pressure is needed only over a small portion of the stroke. It has been common to provide the main ram with a plurality of separate cylinders aligned about the ram to move the ram during the non-compression portions of the stroke. As will be understood by those familiar with hydraulic systems, the pressure and flow of the hydraulic fluid in such systems is generally unbalanced, so that on the compression stroke the pump may be required to provide ten times the volume of pressurized fluid as is required on the retraction stroke. This obviously leads to heat dissipation problems and places a severe limitation on pump requirements in the system. Also, the use of auxiliary cylinders as slaves or positioning cylinders requires additional space, maintenance and installation expense.
In U.S. Pat. No. 3,949,650 Blatt et al disclosed a cylinder construction wherein a concentric inner and outer chamber were formed with the inner chamber receiving pressurized fluid during the compression stroke and the outer cylinder receiving pressurized fluid during the retraction stroke. The area of the inner and outer cylinders were equal, thus the demands on the pump were somewhat lessened, however the ram sacrificed some of its compression capacity to achieve this result. That is to say, a 4" ram built in accordance with Blatt's teachings would be about the size of a 51/2" ram of conventional design and would have a total compression force of only one-half the conventional ram. Also the Blatt et al design does not indicate any recognition of the need to move the rod outside the compression region.